1. Field of the Invention
This invention relates generally to foams made from thermoplastic elastomer (TPE) materials. The invention more specifically relates to microcellular TPE foam compositions, and articles therefrom, wherein the TPE are thermoplastic vulcanizates.
2. Description of the Prior Art
Dynamically vulcanized thermoplastic elastomers (thermoplastic vulcanizates) have a combination of both thermoplastic and elastic properties. Such thermoplastic vulcanizates are prepared by mixing and shearing a thermoplastic polymer, a vulcanizable rubber and a curing agent. The vulcanizable rubber is cured and is intimately and uniformly dispersed as a particulate phase within a continuous phase of the thermoplastic polymer. Early work with the vulcanization of the elastomer phase is found in U.S. Pat. No. 3,037,954 which discloses static vulcanization, as well as dynamic vulcanization wherein a vulcanizable elastomer is dispersed into a resinous thermoplastic polymer and the elastomer is cured (crosslinked) while continuously mixing and shearing the blend. U.S. Pat. No. 4,130,535 discloses thermoplastic elastomer compositions comprising thermoplastic olefin resins and olefin copolymer rubbers, prepared by dynamic vulcanization and wherein the rubber component is vulcanized to the extent that it is essentially insoluble in conventional rubber solvents.
Microcellular foams made from thermoplastic compositions are well known. For example, U.S. Pat. No. 6,231,942 describes both method and apparatus for microcellular polypropylene extrusion, and polypropylene articles produced thereby a polymeric microcellular foam article is comprised mainly of homopolymeric polypropylene, or unimodal polypropylene.
Thermoplastic elastomers have been made into microcellular foams as well. U.S. Pat. No. 6,303,666 B1 describes a process for the production of expanded olefinic thermoplastic elastomer product having good external appearance, flexibility and heat resistance. The process includes a gas-dissolving step, adding carbon dioxide to a molten olefinic thermoplastic elastomer selected from specified olefinic thermoplastic elastomers in a proportion of 0.1 to 30 parts by weight of said carbon dioxide per 100 parts by weight of said molten olefinic thermoplastic elastomer, and forming a molten olefinic thermoplastic elastomer composition in which said olefinic thermoplastic elastomer and said carbon dioxide are in a mutually-dissolved state; and as a cooling step, lowering said molten olefinic thermoplastic elastomer composition in temperature. One class of suitable elastomers is a crosslinked product obtained by dynamically heat-treating in the presence of a crosslinking agent a mixture composed of (a) a crosslinking olefin copolymer, which is an ethylene-α-olefin copolymer composed of ethylene and an α-olefin having 3 to 20 carbon atoms or which is an ethylene-α-olefin-non-conjugated diene copolymer composed of ethylene, an α-olefin having 3 to 20 carbon atoms and a non-conjugated diene, and (b) a decomposable olefin plastic, which is a homopolymer or copolymer in which a content of an α-olefin having 3 to 20 carbon atoms is from 50 to 100 mole %.
U.S. Pat. No. 6,613,811 describes microcellular thermoplastic elastomeric polymeric structures. The articles have an average cell size of less than 100 μm and a compression set ranging from less than about 30% to less than about 5%, and a rebound value of at least 50%. The articles may be formed from a thermoplastic elastomeric polyolefin, preferably metallocene-catalyzed polyethylene.
U.S. Pat. No. 6,713,520 B2 describes thermoplastic vulcanizate foam compositions comprising a mixture that includes from about 15 to about 95 percent by weight of the rubber and from about 5 to about 85 percent by weight of a thermoplastic component based upon the total weight of the rubber and thermoplastic component combined, where the thermoplastic component includes from about 65 to about 90 percent by weight of a conventional thermoplastic resin and from about 10 to about 35 percent by weight of a random propylene copolymer based upon the total weight of the thermoplastic component.
WO 2004/016679A2 describes soft thermoplastic vulcanizate foams comprising polyolefin thermoplastic resin, an at least partially crosslinked olefinic elastomer, hydrogenated styrenic block copolymer, and optional additives. The soft foams have smooth surfaces, low water absorption, improved compression set and compression load deflection.
US 20040115418 A1 describes foams with low water absorption, along with thermoplastic elastomeric (“TPE”) foam materials and methods of forming the same. In some embodiments, the TPE foams have a low water absorption. Microcellular foams are included. Preferably, the TPE materials include polyolefin thermoplastic vulcanizates (TPVs).
In addition to the above, foamed profiles of such rubbers as ethylene-propylene-diene (EPDM) rubber has been used in vulcanized form as weather seals for the automotive industry, for example, where the object is to reduce road noise, dust, grit, and moisture intake at the various openings, such as window seals, door seals, and trunk seals. The elastomeric characteristics of the EPDM rubber foam allow it to conform to the shapes needed and to be effectively compressed into gaps and corners at the openings of the automotive openings when they are closed such that compressed foam hinders the entry of the noise, dust and moisture. However, the construction of the EPDM rubber foam profiles and vulcanization of the EPDM requires careful and difficult handling. On the other hand, TPV compositions are thermoplastic with a pre-cross-linked rubber phases, e.g., EPDM rubber, and can be much more readily formed into complex shapes as with thermoplastic molding, and retain mechanical strength much longer, and still provide resistance to moisture intake, as well as noise, dirt, etc. However, known TPV foam compositions tend not to provide the level of moisture intake prevention that the EPDM rubber foam compound counterparts do.